The front fork assembly of a bicycle is rotatably mounted in the head tube of the bicycle frame and carries and connects the handle bars to the front wheel assembly of the bicycle. A conventional front fork for a bicycle comprises a pair of blades connected to a steerer tube by a crown component. A composite fork of steel and aluminum parts is generally preferred over a fork made entirely of steel parts because all steel forks are generally too heavy to use on a high performance road or racing bike. The conventional fork therefore may be a composite of different metals, such as a hollow steel steerer tube connected to a pair of hollow aluminum blades by a solid aluminum crown as illustrated in FIG. 1 of the drawings. The different components of these conventional fork assemblies may be secured together by swaging, shrink fitting, welding, adhesive or the like.
The crown component may be an inverted Y-shaped structure with depending legs, the lower portions of which are each inserted in a socket at the upper end of a corresponding blade. The base of this inverted Y-shaped crown is connected to the steerer tube. In other conventional fork assemblies, the lower end portion of each crown leg may comprise a socket instead of a projection and the upper end portion of the corresponding blade fits within this socket. In these alternative prior art embodiments, the crown length CL and the transition length TL are generally about the same as for the conventional structure illustrated in FIG. 1.
A light weight all steel fork may weigh about 1.5 lbs. at a minimum. The assembled parts of a composite steel/aluminum fork, wherein the steel pieces are replaced by aluminum pieces with the exception of the steel steerer tube, may weigh between 1.2 and 1.3 lbs. at a minimum. In both, the legs of the crown may be designed to fit down into sockets at the upper ends of the fork blades or the upper ends of the fork blades may be designed to fit up into sockets at the lower ends of the crown legs. The crown of a composite aluminum/steel fork may be a solid aluminum forging with a small hole therein to allow water to drain from the inside of the steerer tube.
Both in the all-steel fork and in the composite steel/aluminum fork, the base of the steel steerer tube is required to withstand the high bending loads to which a front fork is subjected in response to the different forces encountered by a bicycle as it is ridden. The steerer tube therefore is a critical link between the front wheel and the bicycle frame. For this reason, the wall thickness of the steerer tube is sometimes increased significantly, but this adds appreciable weight to the fork assembly. In addition, one important design consideration in making the steerer tube is that it must interface with conventional handle bar stems which require a standard internal tube diameter. This may prohibit the use of steerer tubes with a thicker wall, at least at the handle bar interface, in a front fork designed for industry wide use.
The crown is also a critical link between the front wheel and the bicycle frame because it must take the relatively high loads from the blades and transfer them to the steerer tube through relatively small cross sectional areas. These relatively small areas are at the joints between the different fork components and result in very high stress loadings across these joints. Many prior art forks tend to fail either at the interface of the blades and the crown or at the interface of the crown and the steel steerer tube. Because of the small interface area between the crown structure and the cylindrical base found on conventional steerer tubes, the levels of stress in this interface area is greatly increased over average stress levels in the fork and therefore prior arts forks often fail in or adjacent to this interface area. Stress concentrations in these interface areas may be further aggravated because of the difference in materials where a resin adhesive, a brazing flux or a part made of another metal meets the steel of a steerer tube at the interface. The crown piece therefore transitions the stress loadings from the fork blades to the steerer tube and, thus, serves as a critical link in the load path from the front wheel to the steerer tube. These stress loadings are then passed by the steerer tube to the head tube of the bicycle frame.
In conventional metal forks of the foregoing types, the multiplicity of connections required between the blades and the crown on the one hand and the crown and the steerer tube on the other hand may be made by welding, braising, adhesive compositions, or friction joints such as swaging or shrink fitting. Welding and braising add undesirable weight to the fork. Adhesive compositions for joining different metal parts have significantly different compliance characteristics and thermal expansion coefficients from metal, and therefore often result in excessive concentrations of stresses in the adhesive layer, which in turn may lead to early joint failure. Friction joints also have stress concentrations at the interfaces between parts and generally are not sufficiently strong to sustain the high loads which must be transferred from blade to crown and from crown to steerer tube. It is also difficult, time consuming and expensive to provide metal blades with the cross-sectional shapes required for aesthetic and/or aerodynamic styling.
Prior efforts have been made to address at least some of the foregoing problems with forks made of composite materials comprising a resin reinforced by a fibrous material. For example, U.S. Pat. No. 4,828,285 to Foret, et al., suggests a bicycle fork in the form of a molded, one piece assembly wherein the pivot, the fork head and the two blades are formed by winding textile strands around a previously formed core having the configuration of the fork to be produced and then impregnating the wound strands with an injected resin composition.
While forks of the Foret, et al., type may provide some advantages over conventional bicycle forks, such as improved stress distribution and higher fore and aft strength in the blade to crown area, they have a number of deficiencies. For example, a steel steerer tube generally has better strength, stiffness and performance characteristics than a resin and fiber steerer tube, and metal provides a better bearing surface than do such composite materials. In this regard, the Foret, et al., disclosure does not specifically address the provision of bearing surfaces between the composite fork described and the head tube of a conventional bicycle frame. In addition, tests have indicated that bicycle forks made entirely of textile reinforced resin and having a foam core may have poor compliance, torsional and other performance characteristics when used in combination with a conventional bicycle frame. The core materials around which the Foret, et al., fork is made and which are left in place also add significantly to the weight of the resulting fork structure.
The method by which the Foret, et al., fork is made also has a number of deficiencies. For example, the construction method requires winding numerous lengths of yarn around a rigid or semi-rigid polyurethane core and does not appear to be readily adaptable to efficient mass-production techniques. In addition, since the strands of yarn are wound dry and the fully wound core is then placed into a mold, the resin must be injected into the mold under high pressure. This resin transfer molding (RTM) process has at least two principal drawbacks, namely, it is difficult to achieve a high fiber volume because high fiber percentages block uniform distribution of the injected resin, and the foam core around which the fiber strands are wrapped must have an unusually high compressive strength to prevent its collapse under the high resin injection pressure. Although special high density foams of the necessary compressive strengths are available, these increase both the weight of the product and the cost of its manufacture.